Calculating average time between dates in SQL - mysql

Question

Using MySQL, I'm trying to figure out how to answer the question: What is the average number of months between users creating their Nth project?
Expected result:
| project count | Average # months |
| 1 | 0 | # On average, it took 0 months to create the first project (nothing to compare to)
| 2 | 12 | # On average, it takes a user 12 months to create their second project
| 3 | 3 | # On average, it takes a user 3 months to create their third project
My MySQL table represents projects created by users. The table can be summarized as:
| user_id | project created at |
|---------|--------------------|
| 1 | Jan 1, 2020 1:00 pm|
| 1 | Feb 2, 2020 3:45 am|
| 1 | Nov 6, 2020 0:01 am|
| 1 | Mar 4, 2021 5:01 pm|
|------------------------------|
| 2 | Another timestamp |
| 2 | Another timestamp |
| 2 | Another timestamp |
| 2 | Another timestamp |
| 2 | Another timestamp |
| 2 | Another timestamp |
|------------------------------|
| ... | Another timestamp |
| ... | Another timestamp |
Some users will have one project while some may have hundreds.
Edit: Current Implementation
with
paid_self_serve_projects_presentation as (
select
`Paid Projects`.owner_email
`Owner Email`,
row_number() over (partition by `Paid Projects`.owner_uuid order by created_at)
`Project Count`,
day(`Paid Projects`.created_at)
`Created Day`,
month(`Paid Projects`.created_at)
`Created Month`,
year(`Paid Projects`.created_at)
`Created Year`,
`Paid Projects`.created_at
`Created`
from self_service_paid_projects as `Paid Projects`
order by `Paid Projects`.owner_uuid, `Paid Projects`.created_at
)
select `Projects`.* from paid_self_serve_projects_presentation as `Projects`

Answer 1

You can use window functions. I am thinking row_number() to enumerate the projects of each user ordered by creation date, and lag() to get the date when the previous project was created:
select rn, avg(datediff(created_at, lag_created_at)) avg_diff_days
from (
select t.*,
row_number() over(partition by user_id order by created_at) rn,
lag(created_at, 1, created_at) over(partition by user_id order by created_at) lag_created_at
from mytable t
) t
group by rn
This gives you the average difference in days, which is somehow more accurates that months. If you really want months, then use timestampdiff(month, lag_created_at, created_at) instead of datediff() - but be aware that the function returns an integer value, hence there is a loss of precision.